Process for thermal plasma spraying of doped semiconductor oxide-based coatings

ABSTRACT

A process for the preparation of a composite multilayer product with selective optical properties comprises the operation of depositing, by thermal plasma spraying of powders, a coating of semiconductor oxides doped with oxides of elements chosen from the group consisting of transition metals and metals in the groups III-A and V-A of the periodic table of elements, in which the coating obtained maintains the composition and doping of the thermal plasma sprayed powders substantially unchanged. The figure shows the X-ray diffraction spectrum of a coating deposited according to the process of the present invention.

[0001] The present invention refers to the sector of preparation of products with low emissivity in the infrared field and low reflectivity in the visible field, with application in the aeronautical and energy sectors.

[0002] As is known, in the case of coated products, the coating in question must have a high adhesion and must be able to withstand transitory mechanical-thermal stress that could cause detachment.

[0003] A solution currently adopted is based on special paints containing particles of material with the required optical behaviour. However, the use of said paints is not considered wholly satisfactory. In fact, when exposed to service conditions, these paints tend to rapidly deteriorate and erode and consequently, their properties tend to rapidly degrade. These paints therefore have to be frequently renewed. An example of the above type of coating is described in the U.S. Pat. No. 5,472,533.

[0004] Evolution of the state of the art has brought to light another type of coating: (doped) semiconductor oxide coating.

[0005] The deposit of these oxides has, however, been confined to methods that operate at relatively low temperatures, such as the PVD (Phase Vapour Deposition) evaporation technology, in the Sputtering version.

[0006] In this specific sector, in fact, there is the technical preconception that by adopting thermal plasma spraying, with work temperatures around 10,000° K, the composition and doping of the coating obtained are different from those of the original powder chosen for the thermal plasma spraying.

[0007] The present invention overcomes this technical preconception, also offering further advantages that will become evident below.

[0008] The present invention concerns a process for the preparation of a composite multilayer product with selective optical properties, comprising the stage of depositing, by thermal plasma spraying of powders, a semiconductor oxide coating doped with oxides of elements chosen from the transition metals and metals in the groups III-A and V-A of the periodic table of elements, and in which the coating obtained maintains the composition and doping of the thermal plasma spray powders substantially unchanged.

[0009] The coatings deposited can have a composition chosen from the group consisting of indium oxide doped with stannous oxide and/or cadmium oxide, zinc oxide doped with aluminium oxide and/or indium oxide, stannous oxide doped with antimony oxide and/or zinc oxide and/or fluorine, cadmium oxide doped with stannous oxide and their combinations.

[0010] In particular the coatings deposited can have a composition chosen from the group consisting in indium oxide doped up to 15% with stannous oxide and/or up to 5% with cadmium oxide, zinc oxide doped up to 5% with aluminium oxide and/or up to 5% with indium oxide, stannous oxide doped up to 10% with antimony oxide and/or up to 35% with zinc oxide and/or with fluorine as the F/Sn ratio is between 0.15 and 0.30, and cadmium oxide doped up to 35% with stannous oxide.

[0011] In some embodiments of the present invention, coatings are deposited with a composition chosen from the group consisting of indium oxide doped With 10% stannous oxide (ITO), indium oxide doped with cadmium oxide (ICO), zinc oxide doped with 2% aluminium oxide (AZO), zinc oxide doped with indium oxide (IZO), stannous oxide doped with approximately 3% antimony oxide (ATO), stannous oxide doped with percentages of zinc oxide ranging from 10 to 35% (ZTO), stannous oxide doped with fluorine according to a ratio F:Sn=0.25 (FTO) and cadmium oxide doped with 33% stannous oxide (CTO).

[0012] The low density structural material, on which the deposit is performed, can be chosen from the group consisting of aluminium, aluminium alloys, titanium, titanium alloys, composite products with polymeric matrix or composite products with carbon fibres.

[0013] The composite product with carbon fibres can be of the C—C or C—SiC type.

[0014] The low density high thermal resistance material on which the deposit is performed can be a composite product with carbon fibres. The composite product with carbon fibres can be of the C—C or C—SiC type.

[0015] The high thermal resistance structural material on which the deposit is performed can be chosen from the group consisting of steel and superalloys of nickel and cobalt.

[0016] According to the process of the present invention, an intermediate layer can be deposited between the base and the doped semiconductor oxide coating.

[0017] The intermediate layer can be chosen from the group comprising metals such as titanium, nickel, aluminium, chromium, iron, cobalt, silicon, yttrium and their respective alloys, oxides of metals such as titanium, nickel, aluminium, chromium, iron, cobalt, silicon, yttrium, if necessary doped with elements such as tin, aluminium, antimony, fluorine, and combinations of said metals and oxides.

[0018] The main advantage of the process according to the present invention lies in the fact that it permits the deposition of material with the required optical properties, able to weld permanently to its support.

[0019] The composite multilayer product obtained with the process according to the invention has a number of uses.

[0020] For example it can be used when it is necessary to limit the transmission of heat by radiation, as with machine components, mechanical or electronic devices and instruments which, during their working life, are subject to heating. In these cases a high emissivity in the infrared field could affect the operation of devices and instruments adjacent to those subject to heating.

[0021] Use of these composite multilayer products is also envisaged in situations where it is necessary to prevent, especially in restricted spaces, the onset of the greenhouse effect, determined by solar and artificial lighting, accompanied by an uncontrolled increase in the temperature in that area.

[0022] Another sector of application concerns components which, in their working life, are subject to friction, in particular friction with the air. This occurs on the outer surface of vehicles or their parts that move fast in the air and are subject to friction. The local increase in temperature can determine a high emission of waves in the infrared field and can disturb adjacent electric or electronic instruments and/or permit identification of the vehicle by surveillance systems and/or missile guide tracking systems.

[0023] In the above context, the need to obtain low emissivity of the infrared field can be accompanied by the need to obtain low reflectivity in the visible field near the infrared field, for example to prevent the reflection of external sources, natural or artificial, contributing to identification of the vehicle by surveillance systems and/or missile guide tracking systems.

[0024] Application of the composite product according to the invention is particularly suitable for the surfaces of an aircraft most subject to aerodynamic heating such as the connection edges of the wing and control surfaces, the front of the fuselage and the air intakes.

[0025] With the composite product obtained by means of the process of the present invention, solar energy converters can also be produced for applications in the building trade (walls that retain the heat) or in the field of energy production.

[0026] So far a general description of the present invention has been provided. With the help of the figures and the examples, a more detailed description will now be given of its forms of embodiment for a better understanding of its purposes, characteristics, advantages and modes of application.

[0027]FIG. 1 shows a micrograph of 3% zinc oxide in Al₂O₃ after thermal treatment.

[0028]FIGS. 2 and 3 show two micrographs of 3% zinc oxide in Al₂O₃ sprayed in air, at two different magnifications.

[0029]FIGS. 4 and 5 show the X-ray crystallograms of 3% and 22% zinc oxide in Al₂O₃ respectively, after thermal treatment.

[0030]FIGS. 6 and 7 show the X-ray crystallograms of the coatings obtained by air thermal spraying, on steel substrates, of the powders of 3% and 22% zinc oxide in Al₂O₃ respectively.

[0031]FIG. 8 shows the emissivity in the infrared field of the same two samples of coating compared with the emissivity of the non-doped zinc oxide.

EXAMPLE

[0032] In the example zinc oxide doped with aluminium oxide is used as the coating material. This material is not expensive and its optical properties are well-known. Its reflectivity depends on the concentration of aluminium atoms present in the ZnO reticulum.

[0033] In preparation of the AZO, Al₂O₃ was used instead of Al to avoid problems that can occur with the use of a fine pyrophoric metallic powder.

[0034] Two different concentrations of Al₂O₃ in the ZnO matrix were investigated: the 3% concentration in weight, because the corresponding Al content has very low emissivity values in the infrared field, and the 22% concentration in weight, because this percentage corresponds to the eutectic composition in the ZnO—Al₂O₃ phase diagram (the presence of the eutectic composition guarantees much better uniformity of the doped coating, since it forms during thermal plasma spraying while the materials are completely melted).

[0035] The coating materials to be sprayed are prepared by mixing in water, until a sludge forms, fine powders of ZnO and Al₂O₃ with mean grain diameter of 1.0 μm and 0.5 μm respectively. The mixtures were then agglomerated using a Spray Dryer.

[0036] The agglomeration parameters are shown in Table 1. TABLE 1 Agglomeration parameters Sludge parameters Spray Drying parameters binder (g/l) 40 blower (%) 100 dispersion means 30 compressor (%) 50 (g/l) H₂O (g) 1000 pump (%) 30 ZnO + Al₂O₃ (g/l) 1000 int. temperature (° C.) 250 ext. temperature (° C.) 150

[0037] The agglomerated powders were treated thermally at 1300° C. for six hours.

[0038] For the deposition experiments an apparatus is used comprising an 80 kW plasma torch installed in a pressure chamber which permits control of the pressure and composition of the atmosphere during spraying.

[0039] The coating materials are sprayed, after thermal treatment, on steel substrates.

[0040] Deposit on steel substrates isperformed in APS and IPS (Inert gas Plasma Spray), using the parameters shown in Table 2. TABLE 2 Spraying parameters used for deposit on steel substrates Atmosphere Power Plasma gas Transport gas Mode (bar) (W) (slpm*) (slpm*) APS Air 54 Ar (55) H₂ Ar (3.5) (14) IPS Ar/900 bar 54 Ar (55) H₂ Ar (3.5) (14)

[0041] Samples of powder, incorporated in resin, were observed by means of SEM (Scanning Electron Microscopy). Distribution of the elements was analysed by means of EDS (Energy Dispersion Spectroscopy) and EPMA (Electron Microprobe Analysis). The transverse sections of coated samples were also observed via SEM, EDS and EPMA.

[0042] XRD (X-ray diffraction) analyses were performed on the samples of powders and coatings.

[0043] The surface of some coated samples was ground.

[0044] The reflectivity of the coated and ground samples was measured in the range 0.3-2.5 μm by means of a spectrophotometer. The spectral emissivity of the same samples was measured in the range 1.6-15.0 μm by means of direct comparison with the energy emitted by a black body at the same temperature. To compare the results obtained with the two methods, the emissivity in the range 0.3-2.5 μm was calculated by means of the following relation, valid for non-transparent bodies:

ε+R=1

[0045] where ε is the emissivity and R the reflectivity.

[0046] The samples of powder and coating were identified in the following figures and tables according to the nominal starting composition and the process stage. The agglomerated powders show a mean grain diameter of approximately 80 μm, which remains substantially unchanged after thermal treatment.

[0047] The SEM analyses show that ZnO and Al₂O₃ are well mixed in the agglomerated powders. After thermal treatment, grains up to a diameter of 10 μm can be observed inside individual particles.

[0048] As an example, FIG. 1 shows a micrograph of powder with 3% Al₂O₃, after thermal treatment. The EDS analyses reveal that the white grains observed inside the particles consist basically of ZnO; the slightly grey phase is a mixed zinc and aluminium oxide.

[0049] As an example, FIGS. 2 and 3 show two micrographs of coating with 3% Al₂O₃ sprayed in air, at two different magnifications. The high magnification micrograph shows the same morphology as the thermally treated powder, i.e. white grains in a light grey matrix.

[0050] The XRD spectrums show that ZnO and α-Al₂O₃ are present in the agglomerated powder. After thermal treatment, the ZnO and ZnAl₂O₄ phases are present. The peaks of α-Al₂O₃ have disappeared. As an example, FIGS. 4 and 5 show the XRD spectrums of ZnO at 3% in Al₂O₃ and at 22% in Al₂O₃, respectively, after thermal treatment.

[0051]FIGS. 6 and 7 show the XRD spectrums of the coatings obtained by spraying in air, on steel substrates, the coating materials obtained respectively from the mixtures at 3% in Al₂O₃ and at 22% in Al₂O₃.

[0052]FIG. 8 shows the emissivity in the infrared field of the same two samples of coating compared with the emissivity of the non-doped zinc oxide. 

1. Process for the preparation of a composite multilayer product with selective optical properties, characterised in that it comprises the stage of depositing by thermal plasma spraying of powders, on a base made of structural material with low density and/or high thermal resistance, a coating of semiconductor oxides, doped with oxides of elements chosen from the transition metals and metals in groups III-A and V-A of the periodic table of elements so that the coating obtained maintains the composition and doping of the thermal plasma sprayed powders substantially unchanged.
 2. Process for the preparation of a composite multilayer product with selective optical properties as in claim 1, in which coatings with a composition chosen from the following are deposited: indium oxide doped with stannous oxide and/or cadmium oxide, zinc oxide doped with aluminium oxide and/or indium oxide, stannous oxide doped with antimony oxide and/or zinc oxide and/or fluorine, cadmium oxide doped with stannous oxide, and their combinations.
 3. Process for the preparation of a composite multilayer product with selective optical properties as in claim 2, in which coatings with a composition chosen from the following are deposited: indium oxide doped up to 15% with stannous oxide and/or up to 5% with cadmium oxide, zinc oxide doped up to 5% with aluminium oxide and/or up to 5% with indium oxide, stannous oxide doped up to 10% with antimony oxide and/or up to 35% with zinc oxide and/or with fluorine since the F/Sn ratio is between 0.15 and 0.30, and cadmium oxide doped up to 35% with stannous oxide.
 4. Process for the preparation of a composite multilayer product with selective optical properties as in claim 3, in which coatings with a composition chosen from the following are deposited: indium oxide doped with 10% stannous oxide, indium oxide doped with cadmium oxide, zinc oxide doped with 2% aluminium oxide, zinc oxide doped with indium oxide, stannous oxide doped with approximately 3% antimony oxide, stannous oxide doped with percentages of zinc oxide from 10 to 35%, stannous oxide doped with fluorine according to a ratio F/Sn=0.25, and cadmium oxide doped with 33% stannous oxide.
 5. Process for the preparation of a composite multilayer product with selective optical properties as in claim 1, in which said low density structural material is chosen from the group comprising aluminium, aluminium alloys, titanium, titanium alloys, composite products with polymeric matrix, composite products with carbon fibres.
 6. Process for the preparation of a composite multilayer product with selective optical properties as in claim 5, in which the composite product with carbon fibre is of the type C—C or C—SiC.
 7. Process for the preparation of a composite multilayer product with selective optical properties as in claim 1, in which the material with low density and/or high thermal resistance is a composite product with carbon fibres.
 8. Process for the preparation of a composite multilayer product with selective optical properties as in claim 7, in which the composite product with carbon fibres is of the type C—C or C—SiC.
 9. Process for the preparation of a composite multilayer product with selective optical properties as in claim 1, in which the structural material with high thermal resistance is chosen from the group comprising steel and superalloys of nickel and cobalt.
 10. Process for the preparation of a composite multilayer product with selective optical properties as in claim 1, in which at least one intermediate layer is deposited between the base and the doped semiconductor oxide coating.
 11. Process for the preparation of a composite multilayer product with selective optical properties as in claim 10, in which at least one intermediate layer is deposited chosen from the group comprising metals, such as titanium, nickel, aluminium, chromium, iron, cobalt, silicon, yttrium and their respective alloys, oxides of metals such as indium, cadmium, zinc, tin, aluminium, titanium and tungsten, if necessary doped with elements such as tin, aluminium, antimony and fluorine, and combinations of said metals and oxides. 